Nuclear Backscatter Methodology for flat roof inspections is a non-destructive testing technique uses a radioactive isotope consisting of Americium-241 with a beryllium target to emit high-speed neutrons aimed at the roof. The measurement method relies on the thermalization (slowing) of fast neutrons by the hydrogen atoms in water. Because water contains hydrogen, reflected neutrons indicate the presence of water in the roofing material.

The meter used, Troxler 3216, is a portable instrument with a periodic counter to measure the rate of thermalization of the neutrons.

Since other hydrogen bearing materials also thermalize neutrons, a measurement survey is necessary to establish a relative base level before an analysis can be performed.  A measured roof drawing is rendered and a 5’ by 5’ or 10’ by 10’ grid pattern is established on the roof.

A number of readings are then performed to establish a relative base level before the actual analysis is performed. Testing intervals are typically 7.5 seconds at each grid point location.

Higher levels of slowed neutrons are recorded at wet areas because water contains a significant amount of hydrogen atoms. Although the meter cannot determine the quantity of moisture present in materials, it can easily determine the difference between wet and dry insulation.

This type of testing is often used to augment an infrared inspection because of its ability to “read” down approximately 8 inches and report the relative number of hydrogen atoms.  If water were able to get into the top roof and travel below the insulation, it would be invisible to the infrared camera.